1. Field of the Invention
The invention relates to a power output apparatus, a vehicle including the power output apparatus, and a control unit and method for the power output apparatus.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. JP-A-2004-255901 describes a power output apparatus mounted in a vehicle. In the power output apparatus, a first motor, an engine, and an output shaft connected to wheels are connected to a sun gear, a carrier, and a ring gear of a planetary gear set, respectively, and a second motor is connected to the output shaft via a transmission that includes two brakes. When the engine of the described vehicle is started while the vehicle is at a standstill, the engine is cranked by the first motor with a reaction torque output to the output shaft while the two brakes are both applied to fix the rotational speed of the output shaft to zero. Thus, the vehicle is prevented from moving when the engine is started.
Some of such power output apparatuses make, based on the rotational speed of the output shaft and the rotational speed of the second motor, a synchronous determination for smoothly changing the gears of the transmission and a determination as to whether the second motor and the output shaft are disconnected from each other, that is, whether the both brakes of the transmission are released. If a malfunction occurs in a sensor that detects the rotational speed of the output shaft or a sensor that detects the rotational speed of the second motor, the synchronous determination and the determination as to whether the second motor and the output shaft are disconnected from each other may not be made appropriately. In such a case, the components of the transmission, such as the brake, may wear out because the gears of the transmission cannot be changed smoothly. The rotational speed of the output shaft may change unexpectedly, because the engine is cranked by the first motor with the reaction torque output to the output shaft while the second motor and the output shaft are disconnected from each other. The rotational speed of the second motor may abruptly increase because the torque is output from the second motor while the second motor and the output shaft are disconnected from each other. However, such inconvenience should be minimized.